DIAGNOSIS Audio quality issues are almost always tied to poor or congested Wi-Fi upload speed from your camera to the internet.
FIX Check the camera's RSSI in Device Health. If it's poor, improve your Wi-Fi signal. Also, during the call, avoid using other bandwidth-heavy apps (like streaming video) on your home network. Try talking from the Live View screen on your phone with Wi-Fi turned off, using your cellular data—if the audio is clear then, the problem is your home internet upload, not the camera's mic.
IF THAT DIDN'T WORK There's a known firmware/software glitch that can cause this after some updates. In the Ring app, try disabling and then re-enabling 'Audio Streaming' in the Device Settings. A full power cycle (unplug the camera for 30 seconds, then plug back in) can also clear temporary audio buffer issues.
HEADS UP The speaker and mic are on the bottom of the unit. Make sure they aren't obstructed by an eave, spider web, or debris. Cleaning the small mic/speaker grille with a soft brush can sometimes help.